Fast-acting automatic volume control and sync pulse reshaping circuit



July 19, 1949. A. WRIGHT FAST-ACTING AUTOMATIC VOLUME CONTROL AND SYNC PULSE RESHAPING CIRCUIT Filed June 27, 1945 INVENTOR /vo/w/ We/GHf BY Mgwwvf ATTORNEY Patented July 19, 1949 Y FAST-,ACTING AUTOMATIC VOLUME con',- cmon AND sYNcPULsE REsnAmNc cm- C UIT AAntony Wright, Drexel Hill, Pal., assigner to vRadio Corporation of America, a corporation ci' Dela- Applicatiqnsune 27, 1945., serial No. 601,795, y' j This invention relates to an improvement in television receivers' andin one exemplied embodiment 'to receivers used inV connection with telemetering in which the television transmitter is located in an airplane.v

When television image signals are transmitted from airplanes, considerableV diiculty is encountered by reason of the proximity ofV the airplane propellers. The presenceof the propellers, together with their location, causes an undesired amplitude modulation of the radio frequency carrier, and, in order that these undesired amplitude modulations maybe effectively removed so as not to affect the produced image in the receiver, it is necessary to employ va relatively fastacting automatic vvolume control inthe receiver.

The propeller'rotational speeds usually vary between 1,800 and 2,200 revolutionsper minute. When a two'blade Ypropeller is'A employed, an undesired amplitude modulation of from 60 to' 'l2 eliminate the presence of A these bands on the receiver target area or screen, a fast-acting automatic volume control circuit has been employed, the time constant of the automatic volume control circuit being 'so chosen as to permit the automatic volume control touremove undesired`V` modulations up toabout 200 cycles per second.

For the transmission of information by television under such circumstances. it is sometimes convenient to transmit, for vertical synchronizy the vertical or eld or frame sync impulses, as'

well as the blanking impulses, areV materially attenuated by reason of they lfact that the increased carrier amplitude transmitted during the sync and blanking impulse interval is sufliciently long to cause the fast-acting automatic volume control circuit to respond, withthe result that the receiver gain` is appreciablyreduced during the blanking impulse period. By reason of the reduction in gain during the vertical sync and *blanking interval as produced by the automatic volume control circuit, the intensity of the Yvertical sync impulse is materially reduced so that vertical synchronizing frequently becomes difficult.

y VIt is, therefore, the purpose of the present invention to'lprovide a'circut arrangement for use in receivers employing fast-acting automatic volumecontrol circuits whereby the vertical sync impulses may be restored to permit and to assure `proper synchronizing of the vertical deflection generators. A separate blanking pulse, generated in the receiver and derived from the denection pulses which are initiated by the sync pulse, may be used to compensate for the attenuated transmitted pulse; 1

Another purpose of the present invention resides in the provision of a circuit arrangement for assuringproper operation of the vertical deflection generator in a` television receiver, even though the receiver incorporates a relatively fastvacting yautomatic volume control circuit.

Still another purpose of the present invention residesin the provision of means in a television receiver in whichk fast-acting automatic volume control, is employed whereby the vertical sync impulses, even though, attenuated by reason of the Vautomatic volume control, may be .restored by an amount suiicient to assure proper operation of the vertical deflection generator.

Various other purposes and advantages of the present invention vwill become. more apparent to those skilled in the artv from the following detailed description, particularly when considered in connection with the drawing, wherein the single figure thereof shows a schematic diagram of a :preferred form of the present invention.

Referring now to the drawing,` a portion of a complete television receiver is shown which includes an antenna I Il for 'receiving' the transmitted signal energy from the television transmitter. 'This antenna supplies the received signal energy to the4 'portion I2`of the television receiver which includes a radio frequency tuner, the

local oscillator, the mixer and the intermediate frequency amplifier. .'Iheoutput' fromA this portion I2 of the television receiver is applied to a Vdemodulatorrtube I4 which includes an anode I6 `modulated signals are then available from the cathode) I'B of the demodulator tube'.

Vfrom the 'anode 58 of the rectier 54. 'automatic Volume control potentials A'are applied to the portion I2 of the. television receiverrby iie'modulate'd viideor signal. i `conductor 60 and Ithe'an'ode -58 of the automatic includes at least a cathode 30, a control electrode i.

32 and an anode 34, and, although the amplifier tube 28 is shown as a triode'li'ri the eieiit diagram, a high gain screengi'id i" petde tube would normally be employed. After the television image or video signals have. been ampl-iedby the video amplifier (including one or incre tubes such as shown at 28) the amplified signals are then applied to theimage producing tube, for example a tube of the type as' 'the kinescope by way of conductor 36. Y

It will be observed that the cathodes of th demodulator' I 4 andthe limiter 22, aremaintained aty a potential approximately` v3()v volts negative `with respect to ground. 'I Vhis biasing of the cathodes of these two tubes does not .anyway affect the demodulator, but, in View of the direct current path through the grid resistance 13B' -a'ssociated with the control electrode 32 of tube `28; a bias is applied betweenmthec'athode and anode of the limiter tube 2.2; This bias normally permits conduction oi the limitern tube, but when the amplitude of the demodulated image or video signals present at thecathode of the demodulator is sufficiently positive to overcome the bias of Ythe limiter, conductionthrough the limiter ceases with the result that disturbances or Aimpulses that would produce undesired high voltage deviations of thelvideo signals are limited and are prevented from being applied :to the video amplifier. A

vThe limiter, in addition Ito spply'ingdisturbance o r noise c lip'pedvide 'or iiiagey signals 'to the video anriplifierv 2B als sifip'ygl the y'signals t an automatic vllifne cetl amp fier tube l and to ine horizontaler une sync signal separating circuits (not shown) by bi conductor 42. The horizontal 'or line sync signal separator 'Iry be of conventional 'desig'll vand S ectve .ibi 4Teuimiying,the 'required Asi'giiais to the Vbona-omai deflection 'generator for ine iiaage The auioinatienvcimii'e ceiitroi 'anibiiiiei tube 4o includes a cathode u, a control 4electrode le and all anode 43. The eathde is cohr'leeted to eroiiiiqubiie "tbe 'anode `is mainte-incuriosisce with 'respect to ibecaibeee by a Vsource of boei- 'cathode 52 of the automatic volume control recti- -'er 54 by way of a coupling condenser 56. l Automatic volume control potentials are then available W'ay of conductor '6U and are proportional to the voluine control rectier `54 is an intermediate `frequency carrier lter :circuit and a time constant 'circuit -including'resistors 62'and 64 as Well as condensers 66 and 68e By proper choice of the lsufiieiently fast=aeting to effectively remove the i undesired amplitude-modulationsi Wave forms are also shown Avassocate-d Awith lAfter the sign'aiaas VappliedV the operation of the circuit arrangement. The transmitted radio frequency energy is shown associated with the antenna I0 and for the purpose of simplicity, this transmitted radio frequency energy does riotinclude any' undesired amplitude modulations. The wave forin is, therefore, more indicative of the transmitted signal energy than of the actual received signal energy. The curve B lier 28 and to the automatic Volume circuit amplifier 40. At this particular point in the circuit, the sync signals extend in a positive direction, and, for simplicity, thehorizontal or line sync impulses have been omitted. It will be observed, however,

' Athat 'the' wave feiiii of the frame or vertical sync impulse is materially different from the modulation wave form and this change in wave form is by reason ofHtheUfast-acting automatic volume control circuit.v The'steep iront "of the vertical or r'e npul'seis retained since `the alltoinatie vlu e ciitrl circuit is `ineiective to 'remove' 'the iii'gh freqencyjiepresented by the steep front, but the remainder of the vertical 'or frame sync impulse as Iwell 'as the immediately renewing manning impune 'are maieiiauy attenuange 'produced in the effective l er by reason 'of the action rof a'st-'acting 'aut :tic vliiiiiecontrol.

The 'curve :CQ-snows the wave form present at the cathode of the automatic volume control rectiei' vtube 54 and at point in the circuit the 'sync impulses vextefnu in a negative direction, It be observed that a considerable portion 'of tbe bignerfiequency ceibpbiients ci the video di' image signals have l'aeeiiV removed since the circuit parameters associated with the automatic volume control amplifier tube 4B areV riot'particularly designed to pass the higher frequency conponents of thejimage or video signal `since it is'not necessary that these components be present `in any of the circuit arrangements associated with the anode of the automatic volume control ampli- "fier i'iuibe 40.v The steep front of the vertical or frame 'Sync in'i'pulse is still retained but conthereafter witriine result that the steep wave l front for high fi'eilu'ency ic'rnpone'ntwappears in the 'automatic Vvolume control am'pnner anode circuit as an integrated pulse with relative low amplitude. i The automatic volume/ebraici voltage is obtainedro'm the peaks of the sync impunes 'and are effective for determining the effective gain 'ci the portion 1"2 ci 'the te1evisicn receiver. i, i

1n creer' te 're-establish the vertical or 'frame sync impulse to anreiitent such that 'it would be These 60.

the vertiea1 den'ectin generator, vonage variations present ai the anbdebfv the Vaiitoiiiaiic 'temine con 'ainpliner veriei anzi abuelieii by ka sync 'signal aipiier tube 1t. The

. 'voltage variations, representen by tbe curve o,

Located vbetween the Y are app'iieu tje the control electrode 'ci the 'sync signai amplifier and since 'me 'anode 12 ci ibis tube is connected ,to a -silree of positive poteniiai by way-cf aieaai impedance 14, ampuiiedvbitage Yariaticns 'win yibe present 'at the abode tube n.. These vonage variat iis `aie represented by the vcurve D, and, et thi oint 'the circuit, tbe sync Y impunes Varteria are positive direction. VAftertl'ie ,frame or vertical sync 'impi1lses have been suiiiciently'aipl'ineii by the amplifying tube 1o, they linaytheii be applied ib a sync signal the elements inthe diagram'to assist in explaining 75 separator tube Hi which ineludes, in addition to a cathode, a control electrode 18 and an anode 80. The control electrode 18 of the sync signal separator tube 16 is normally maintained at a negative potential so that the separator tube 16 is biased to cut-oil? at approximately a point represented by the dotted line 82 associated with the curve D. Only those signals which extend in a positive direction and having an intensity Sunicient to overcome the bias potential of tube 16 will be effective to produce a voltage variation at the anode 8U of the separator tube. Since the wave front of the vertical sync impulses normally exceeds this bias potential, they produce voltage variations extended in a negative direction at the anode 80 of the separator tube. These voltage variations are represented by the curve E associated with the anode of the separator tube '16. The output from the vertical or frame sync signal separator tube 16 is then applied to the control electrode 82 of the sync signal amplifier tube 84 where their intensity is increased. Amplified vertical sync signals are then available from conductor 86 associated with the anode of the sync signal amplifier tube 84. The wave form of the voltage variations available at conductor 86 is represented by the curve F associated with this conductor, and the vertical sync impulses extend in a positive direction and have sumcient amplitude to cause proper response and operation of a conventional vertical deflection oscillator (not shown).

In view of the foregoing, it will, therefore, be observed that through the use of the present invention reliable operation of the vertical or frame deflection generator is made possible even though the television receiver includes an automatic volume control circuit which is sufliciently fast in its operation to produce considerable attenuation of the sync impulses. In the absence of such a circuit as described above and shown in the drawing, the intensity and effectiveness of the vertical or frame sync signal is not sufficient to produce reliable operation of a conventional vertical deflection generator. When the present invention is incorporated in a television receiver which is designed to operate under the conditions described above, satisfactory operation may be obtained and the television receiver will remain in proper synchronous operation with the transmitter, even though a fast-acting automatic volume control circuit is incorporated in the receiver for the purpose of removing undesired amplitude modulations in the received radio frequency energy.

In the above description it has been assumed that the line deflection is in a horizontal direction across the target electrode of the camera tube and across the screen of the receiver tube, while the field deflection is in a vertical direction. The present invention could be exercised, however, if these deflection directions were reversed. Under such a condition the frame or horizontal sync impulse would be re-established by the present invention.

I claim:

l. A television receiver including means for receiving and demodulating the radio frequency carrier that has been amplitude modulated by television image signals and by horizontal and vertical sync impulses, an amplifier for amplifying the low frequency components of the image signals and the vertical sync impulses as derived from the carrier demodulator, a rectifier for rectifying the amplified low frequency components of the image signals, a circuit arrangement asso- 6 ciated with the rectifier for producing fast-acting automatic volume control potentials for removing undesired modulations of the radio frequency carrier of a predetermined low frequency, the said fast-acting automatic volume control circuit inherently functioning to appreciably attenuate the vertical sync impulses immediately following the leading edges thereof, a further amplifier for amplifying the low frequency components of the image signals and the residual leading edge of the vertical sync impulses, an electron discharge tube having a cathode, a control electrode and an anode, means for maintaining a predetermined average potential difference between the control electrode and the cathode, means for applying the amplified signals to the control electrode of said discharge tube with the residual vertical sync impulses extending in a positive direction whereby said discharge tube will operate to.

separate the amplified leading edge portion of the vertical sync impulses from the image signal, and means for further amplifying the so separated leading edge of the vertical sync impulses.

2. A television receiver adapted to receive a radio frequency carrier that is amplitude modulated by image signals and by horizontal and vertical sync impulses, and in which the carrier may contain, in addition, an appreciable undesired amplitude modulation of a predetermined relatively low frequency, said receiver including a fast-acting automatic volume control circuit for substantially nullifying the effects of the undesired amplitude modulation and in which the fast-acting automatic volume control also results in an appreciable attenuation of the vertical sync impulses immediately following the leading edge portion thereof, comprising an electron discharge tube for amplifying at least the low frequency components of the image signals and the residual leading edges of the vertical sync impulses, means to rectify the amplified low frequency components to produce fast-acting automatic volume control potentials, means to further amplify the low frequency components of the image signals and the residual leading edges of the vertical impulses, means including a second electron discharge tube having a cathode, a control electrode and an anode, means to apply the further amplified low frequency components and the residual leading edges of the vertical sync impulses to the control electrode of said tube with the vertical sync impulses extending in a positive direction, means to bias the control electrode negative with respect to its associated cathode whereby amplified leading edge portion of the vertical sync impulses are separated from the low frequency components of the image signal, and means to further amplify the so separated portion of the vertical sync impulses.

ANTONY WRIGHT.

REFERENCES CITED The following references are of record in the me of this patent:

UNITED STATES PATENTS Number Name Date 2,055,748 Lubcke Sept. 29, 1936 2,240,533 Wilson May 6, 1941 v 2,246,947 Martinelle June 24, 1941 2,313,915 Bedford Mar. 16, 1943 2,332,681 Wendt Oct. 26, 1943 2,402,096 Somers June 11, 1946 2,404,626 Tyler July 23, 1946 

